Question

Assertion: Thin films such as soap bubbles or thin layers of oil on water show beautiful colours when illuminated by white light.
Reason: It happens due to the interference of light reflected from the upper surface of thin film.

A) Both assertion and reason are correct and reason is the correct explanation for assertion.
B) Both assertion and reason are correct but reason is not the correct explanation for assertion.
C) Assertion is correct but reason is incorrect.
D) Assertion is incorrect but reason is correct.

Answer 1

Hint: Use the theory of interference. That superposition of two or more waves gives the resultant wave having more energy. In assertion reason problems, first check if the assertion and reason are true or not. If both the assertion and reason are true then check if there is any relation between the two.

Complete step-by-step answer:
Principle of superposition says that, the resultant displacement of a number of waves in a medium at a particular point is the vector sum of the individual displacements produced by each of the waves at that point. that is ,
\[Y = {y_1} + {y_2} + {y_3} + \ldots \ldots \ldots .\]
It is because of interference of light waves reflected from the upper and lower surfaces of the thin film of soap bubbles. The colored interference fringes are produced due to the fact that the constructive and destructive interference is dependent upon the wavelength of light used.
When this occurs the resultant wave at any point at any instant of time is governed by the principle of superposition.
The phenomenon of redistribution of light energy in medium as a result of the superposition of light waves from two coherent sources is called interference. Any two sources of light continuously emitting light waves having zero or constant phase difference are called coherent sources.

Option A is the correct answer.

Note: In interference there are alternate bright and dark regions that are observed and they are called interference bands or interference fringes. The color pattern observed on soap bubbles and oil spills on wet roads in sun light are due to interference of light. There are two types of interference.

Constructive interference: At some points where the crests of one wave falls upon the crests of the other or the trough of one wave falls on the trough of the other, the resultant amplitude is large and hence the intensity becomes maximum.
Destructive interference: At other points where the crests of one wave falls on the trough of the other wave and vice versa, the resultant amplitude is reduced to zero and hence the intensity is minimum.